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I use www.weatherbase.com for historical weather info. You have to check each location you are visiting, as these countries are impacted by different monsoon systems.

You can certainly spray your clothing with Permethrin if you wish. It is somewhat effective. More important is applying an effective repellant to all exposed skin. You can use either a deet repellant at 20-33% concentration or a picardin repellant. Both are effective.

Exactly where are you going in these three countries? Malarial risk is quite variable across these countries - most of Cambodia is malarial risk, very little of Thailand is malarial risk, only parts of VN are malarial risk. Read up on your risks, and discuss whether to take an anti-malarial with a travel medicine professional.

March is hot and dry in Cambodia. Therefore probably less risk of Malaria than, say, September. My daughter has lived in Siem Reap for the past six years and does not take any anti-malarial medication. I go to Cambodia every year in June - July and have stopped taking anti-malarials. You should be fine. If you are on a high end tour you are unlikely to going too far off the beaten track. Use repellent, especially in the evenings, and cover all exposed skin with sun screen during the day.

I would check the CDC website for the areas you will be visiting. It's not only malaria that you may need to be concerned about, but also Dengue fever. Mosquitos carrying dengue are out during the day. we've been traveling in the countries that you are going. We did take malarone as directed for those areas with malaria.. We also used deet on exposed skin for areas with dengue. We did not spray our clothes. We have not seen a lot of Mosquitos but there were some in Siem Reap, Chiang Mai and Chiang Rai. We are currently in VN now and saw a few Mosquitos in the mountains in the North. It far from the Laos border, but I doubt a high end tour company will be going to these areas.
enjoy your trip!

We are also going on a tour in March of Vietnam and Cambodia with a few days in Hong Kong up front. We used the services of an international travel clinic here in Toronto and the doctor recommended that we take Malarone just before, during, and for a few days after the Siem Reap portion of our journey (but no where else.) He suggested that we try it at home to assess side effects which we have done. We will of course load up on bug spray. The Malarone didn't cause us any problems during our test so we have decided to use it as an additional precaution. We may discontinue the dose immediately after leaving Siem Reap if we haven't had any bites. It may be a lot for nothing but who knows...

You must take Malarone daily for a week after leaving the malarial risk area in order for it to be effective. Mrs. Wally, if you discontinue it after leaving Siem Reap you'll have exactly the same protection against malaria if you has not taken an anti-malarial at all. While Siem Reap itself is considered low risk for malaria, there is malaria in the rural areas around Siem Reap and at the surrounding temples.